The use of so-called magnetic chucks for retaining workpieces upon machine tools is widespread, especially in surface grinding, milling and drilling applications, such chucks being switched between an "effective" or ON state and an "ineffective" or OFF state by mechanical or electrical means.
In the effective state of the magnetic chuck, a magnetic field appears at a surface against which a magnetically attractable workpiece can be held by this field. In the OFF state, the magnetic field is released or eliminated at this surface so that the workpiece can be removed.
Some background in this field can be found in U.S. Pat. Nos. 3,340,442, 3,775,717 and 4,075,589 as well as in the references cited therein.
The term "magnetic chuck" is used herein in discussing the background of this invention, to refer also to worktables having the same capacity to affix a magnetically attractable workpiece to a surface.
More specifically, it is known to provide (see, for example, the French Pat. No. 1,452,241 commonly owned with the present case) a magnetic device for the purposes described which comprises permanent magnets and can be switched between effective and ineffective states. This device comprises a hollow body in which at least one first magnetic assembly is constituted by two mutually parallel stacks of permanent magnets extending longitudinally and in which the permanent magnets alternate with pole pieces. The polarities of the magnets are such that the pole pieces separate from the housing alternate along the stack between north and south polarities.
At least one second magnetic assembly is provided within the hollow body and is constituted by a stack of permanent magnets parallel to the first stacks and disposed between them, the second stack also having its permanent magnets alternating with pole pieces with alternately north and south polarities along the stack.
The magnetic field strengths of the two assemblies are equivalent so that a longitudinal displacement of one assembly relative to the other in a longitudinal direction through a distance corresponding to one pole width relative to the other assembly, between the effective and ineffective positions, causes the flux contributions of the two assemblies to cancel. A displacement of an equivalent degree in the opposite direction, causes flux addition and provides the desired magnetic field at the surface of the body.
The two spaced-apart stacks of the first assembly are thus stationary while the single stack of the second assembly is disposed centrally of the first stack and is connected at its end to a simple eccentric mechanism which effects the displacement. This mechanism, therefore, permits switching between the effective and ineffective operations.
In the ON position, the north poles of the movable central stack register with or are aligned with the north poles of the lateral stacks and the same applies to the south poles of the two assemblies. The lines of force of the magnetic field thus close through the air along the chuck surface between the opposite poles and permit attachment of magnetically attractable bodies to this surface. The flux of the two assemblies is therefore additive.
In the OFF position the north poles of the central stack are in registry with the south poles of the stationary stacks while the south poles of the movable central stack register with north poles of the stationary stacks. The lines of force of the magnetic field thus close within the hollow body in the juxtaposition regions of the mutually offset stacks thereby shortcircuiting the magnetic field of the stacks and neutralizing the fluxes. This annuls the magnetic field at the critical chuck surface and permits the workpiece to be removed or placed on the surface without impediment.
Since the magnetic forces are symmetrical with respect to the axis of the central stack, the force required to displace the latter is small thereby facilitating manipulation between ON and OFF states and increasing the useful life of the actuator. Since the pole pieces extend transversely of the axis and are of limited thickness, the pole width or pitch of the chuck is small enabling the chuck to be used to hold even relatively small magnetically attractable objects.
However, with the system described, utilizing a hollow body of magnetic material to accommodate the stacks, it is common practice to provide a sole plate to rest upon the worktable, a rectangular frame forming the four lateral faces and an upper pole plate. Neither the frame nor the sole plate participate in magnetic functions. Only the upper pole plate performs a magnetic function and has its attachment surface defined by alternating spacers and pole pieces corresponding in pitch or width to the pole pieces of the stationary stack with which they are aligned.
This construction, while very effective in attaching workpieces to the aforementioned surface is not always fully satisfactory since it cannot be used effectively unless it is attached mechanically by appropriate means, e.g. bolts, to the worktable and is incapable of holding objects which must be positioned by the lateral surfaces of the chuck. In fact, the mere presence of the sole plate appears to limit the versatility of the device.